Ethylene oxide is an important industrial chemical used as a feedstock for making such chemicals as ethylene glycol, ethylene glycol ethers, ethanol amines and detergents. Other industrially important olefin oxides are for example propylene oxide and butadiene oxide. A method for manufacturing an olefin oxide is by the catalyzed partial oxidation of the olefin with oxygen yielding the olefin oxide, which is referred to hereinafter by “olefin epoxidation”. The olefin oxide so manufactured may be reacted with water, an alcohol or an amine to produce a 1,2-diol, a 1,2-diol ether or an alkanol amine.
In generally applied methods of olefin epoxidation, a gaseous feedstream containing the olefin and oxygen is passed over a packed bed of shaped catalyst particles positioned in one or more reactor tubes. The catalyst generally comprises silver on a support. The feedstream is compressed in order to overcome the resistance to flow of the packed bed. During normal operation, the catalyst is subject to an aging-related performance decline. The aging manifests itself by a reduction in the activity of the catalyst. Usually, when a reduction in activity of the catalyst is manifest, the reaction temperature is increased in order to compensate for the reduction in activity. The reaction temperature may be increased until it becomes undesirably high, at which point in time the catalyst is deemed to be at the end of its lifetime and would need to be exchanged.
When the catalyst needs to be exchanged, an opportunity arises to reconsider the conditions of economically optimal operation of the olefin epoxidation process. Such optimal conditions may have changed as the economy of operating the process has changed. For example, the economy may have changed as a result of changes in the values of the olefin and/or oxygen used as components of the feedstream, changes in the value of one or more of the olefin oxide, 1,2-diol, 1,2-diol ether, and alkanol amine products, and/or changes in the value of energy used, for example, for compression of the feedstream. Also for the process to be operated in a new plant, consideration has to be given to the conditions of economically optimal operation.
In one aspect, the consideration of the economically optimal operating conditions involves the balance between, on the one hand achieving the potential for a high productivity by packing a large quantity of shaped catalyst particles in the reactor tubes, and, on the other hand achieving a low pressure difference over the packed bed, that is to minimize compression costs. The quantity of shaped catalyst particles packed in a reactor tube may be expressed as the volume fraction of the packed bed occupied by the catalyst particles or by the packing density. It goes without saying that, generally, packing a larger quantity of shaped catalyst particles in the reactor tube and maintaining the flow rates goes hand-in-hand with a higher pressure difference over the catalyst bed, and, hence with higher compression costs.
Given the dimensions of the packed bed in the reactor tubes and the shape of the catalyst particles, the quantity of shaped catalyst particles packed in the packed bed and the pressure difference over the packed bed may be governed by the dimensions of the catalyst particles. Selecting the dimensions of the shaped catalyst particles such that desired values of the quantity and/or the pressure difference can be accomplished requires an extensive trial and error experimental program.
The selection process and the reasons behind the selection process as described hereinbefore in the context of olefin epoxidation are in an analogous manner applicable to many other processes which involves shaped particles packed in a tube, for example absorption processes, for example, using guard beds; heat exchange processes; and conversion processes other than olefin epoxidation, such as processes for manufacturing maleic acid and vinyl acetate, hydrogenation processes, Fisher-Tropsch synthesis, and catalytic conversion of exhaust gases, for example automotive exhaust gas or industrial exhaust gas.